You could try this...
Let say your gridview is called gvUsers in the RowDataBound method you could try this...
protected void gvUsers_RowDataBound(object sender, GridViewRowEventArgs e)
{
if (e.Row.RowType != DataControlRowType.DataRow) return;
DataRow CurrentRow = ((DataRow)sender);
for (int i = 0; i < CurrentRow.ItemArray.Count(); i++)
if (CurrentRow.ItemArray[i].ToString() == " ")
CurrentRow.ItemArray[i] = string.Empty;
}